The present invention relates to ceramic structures which include an NZP-type phosphate phase and a second phase of a substantially higher heat capacity per unit volume than the phosphate phase, the ceramic structures having a near zero thermal expansion over a temperature range from 22xc2x0 to 1000xc2x0 C., high permeability by virtue of high porosity and large median pore size, and good filtration efficiency, such that the structures are suitable for use as diesel particulate filters.
Recent interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream. In 1998 diesel particulates were declared a toxic air contaminant by the California Air Resources Board. Legislation has been passed that regulates the concentration and particle size of diesel particulate pollution originating from both mobile and stationary sources.
Diesel particulate material is mainly carbon soot. One way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter which filters the diesel exhaust by capturing the soot in or on the porous walls of the filter body. These filters known as xe2x80x9cwall-flowxe2x80x9d filters in which approximately half of the channels are plugged on the entrance face of the filter and the adjacent channels are plugged on the exit face of the filter, have been proven effective in trapping the carbonaceous particles present in the exhaust generated by diesel engines. Such filters typically have about 40-55% porosity and a median pore size of about 8 to 20 micrometers.
Once the carbon in the filter has accumulated to some level, the filter must be regenerated by burning the soot. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600xc2x0 C. to a maximum of about 800-1000xc2x0 C.
The highest temperatures during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter. Under certain unusual circumstances, a so-called xe2x80x9cuncontrolled regenerationxe2x80x9d can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot (a reaction which is already highly exothermic) may produce temperature of up to 1300xc2x0 C.-1500xc2x0 C. or even higher.
In addition to capturing the carbon soot, the filter also traps xe2x80x9cashxe2x80x9d particles, which consist of oxides such as those of calcium, zinc, magnesium, phosphorus, and sulfur, that are carried by the exhaust gas from burning of the engine lubricating oil, and oxides of metals such as iron and cerium that may be present as additives to the diesel fuel to aid in combustion of the soot. Additionally, oxide particles of iron, copper, and zinc may be present from wear of the engine components. These particles are not combustible and, therefore, are not removed during regeneration. However, if temperatures during uncontrolled regenerations are sufficiently high, the ash may eventually sinter to the filter or even react with the filter resulting in partial melting. Thus, filters must periodically be removed from the exhaust system and the ash must be flushed out of the filter, after which the filter is reinstalled.
During uncontrolled regenerations, the highest temperatures tend to occur near the exit face, where concentrations of oxide particles are greatest. These temperatures can result in extensive reaction between the oxide deposits and the ceramic filter. Depending upon the concentration and/or chemical composition of the oxide deposits, sufficiently high temperatures and long enough times at temperature can result in severe corrosion or even partial melting of the filter, producing xe2x80x9cpinholesxe2x80x9d through the walls of the filter. These pinholes subsequently lower the filtration efficiency of the filter by allowing soot to leak through the filter and exit the tailpipe. In sufficient number, the pinholes can result in a filter that is no longer in compliance with government environmental regulations. Alternatively, sintering and densification of the oxide deposits, or of the ceramic filter by reaction with the oxides, may result in reduced permeability and an unacceptable increase in back pressure of the filter against the engine.
Even if the temperatures are not sufficient to damage the filter, they may be high enough to cause partial sintering of the oxide particles to themselves or to the surface of the filter walls. Such sintered ash deposits may be difficult or impossible to remove from the filter during periodic maintenance, resulting again in an increase in back pressure and loss in soot loading capacity, the latter necessitating more frequent regenerations.
In the industry cordierite (2MgO-2Al2O3-5SiO2) has been the cost-effective material of choice for diesel particulate filters for heavy duty vehicles due to its combination of excellent thermal shock resistance, filtration efficiency, and durability under most operating conditions. However, a significant problem associated with conventional cordierite diesel particulate filters is susceptibility to damage during the required filter regeneration cycling.
Pending application having Ser. No. 09/671,722, entitled xe2x80x9cRefractory NZP-type Structures and Method of Making and Using Samexe2x80x9d by Gregory A. Merkel, co-assigned to the present assignee, discloses NZP-type structures of the general composition having the general formula RxZ4P6- ySiyO24, where 0xe2x89xa6xxe2x89xa68, 0xe2x89xa6yxe2x89xa66, R is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, and/or lanthanides, and Z is Zr, Ti, Nb, Ta, Y, and/or lanthanides which possess near zero thermal expansion and melting points greater than 1700xc2x0 C. and are especially useful as diesel particular filters. During laboratory bench tests, 2xe2x80x3 diameter and 6xe2x80x3 long filters of the compositions Ba1.25Zr4P5.5Si0.5O24 and Sr1.3Zr4P5.4Si0.6O24 have loaded with artificial carbon soot and have survived repeated simulated uncontrolled regenerations to 1600xc2x0 C. However, larger-diameter filters ( greater than 5xe2x80x3) when loaded with carbon soot and regenerated on engine dynamometers experienced failure. Examination of these filters after testing had shown that iron oxide containing particles (virtually absent from the laboratory tests using artificial soot), coupled with high temperatures, have resulted in the formation of holes extending all the way through the walls of the filter. The concentration of iron and the number of holes was greatest near the exit end of the inlet channels.
A need therefore exists for an NZP-type phosphate-based ceramic useful for diesel particulate filters absent the shortfalls described.
The present invention provides such a ceramic and a method of making and using it.
The instant invention is founded upon the discovery of a ceramic comprising a first phase having a general formula R1+(x/2)Zr4P6-xSixO24 where R is selected from the group consisting of Ba, Ca, and Sr and 0xe2x89xa6xxe2x89xa62, wherein the first phase has a volumetric heat capacity (Cp1), and at least 10 weight percent of a second phase having a volumetric heat capacity (Cp2), wherein Cp2 greater than Cp1, wherein the ceramic has a coefficient of thermal expansion from 22xc2x0 to 1000xc2x0 C. of xe2x88x9215xc3x9710xe2x88x927/xc2x0 C. to +15xc3x9710xe2x88x927/xc2x0 C., a permeability of at least 0.25xc3x9710xe2x88x9212 m2, an total porosity of at least 35% by volume, and a median pore diameter of at least 6 micrometers, and a volumetric heat capacity of the solid, Cp(solid) of at least 3.15 J/cm3K, wherein the volumetric heat capacity of the solid is equal to the measured heat capacity, in units of J/g K, multiplied by the density of the solid portion of the ceramic in units of g/cm3. The solid portion of the ceramic is exclusive of the porosity of the ceramic.